1. Field of the Invention
The invention relates generally to the field of radars. More particularly, the invention relates to synchronization of multiple radars. Specifically, a preferred implementation of the invention relates to synchronization of multiple radars operating within the same frequency band.
2. Discussion of the Related Art
Lower-frequency radars, operating in the MF, HF, and VHF bands, have a number of uses. The primary application, however, has been the real-time mapping of ocean surface currents and local wave monitoring from coastal stations. For this purpose, these lower frequency radars offer many advantages. (i) With antennas sited at the coast, they have the ability to see beyond the horizon in the surface-wave propagation mode; lower frequencies achieve longer ranges. (ii) Only at these low frequencies can surface currents be extracted from the echo that is Bragg-scattered from the ocean waves; more prevalent microwave radars cannot measure surface currents, and alternative cost-effective measurement technologies do not exist. (iii) Reduced data rates, resulting from the low frequency and unique FMCW signal modulation and processing permit easy, low-cost digital signal generation and processing. The radars considered here may operate typically three orders of magnitude lower in frequency than the more common microwave radars. Over 100 coastal HF radars have been built and now operate for ocean surface current monitoring, and demand for more units is growing rapidly. This number, plus three skywave over-the-horizon radars, constitutes the entire number of all HF radars in the world at present. This contrasts with perhaps 200 million microwave radars built and in existence worldwide.
Because the MF/HF/VHF bands have not been used for most radar applications up to now, there have been no frequency bands designated for radar below 430 MHz, either in the U.S. or worldwide by the ITU (International Telecommunications Union). Thus, users must apply for “secondary licenses”, meaning they cannot interfere with “primary” users. To avoid interference, each user would like a frequency separate from all other HF radar users (as well as from the conventional radio users of these bands). The problem is exacerbated by the wide signal bandwidths needed for radar operation in contrast with radio communications. To achieve a 1 km range cell, one needs 150 kHz bandwidth, for example. Typical radio channels occupy 5 kHz bandwidth or less. This means that one radar user monopolizes 30 potential radio channels. Finally, a given fixed bandwidth (like 150 kHz) occupies a much larger fractional bandwidth percentage-wise at HF (e.g., 5 MHz) than at microwave (e.g., 5 GHz). All of this makes it clear that each new user will not receive a separate frequency for his own use; multiple users must share the same frequency in a manner that does not cause mutual interference.
One way to do this is time multiplexing. By this method, several radar stations would time share a frequency, radiating one at a time in a synchronized fashion. Two types of time multiplexing are possible: station sequencing and pulse-to-pulse interleaving.
In the time-multiplexing method referred to as station sequencing, each radar transmits for a several-minute period on a sequential schedule. This has a major disadvantage for mapping surface currents from sea echo. The quality and accuracy of the vector maps improves as longer echo time-series data sets are processed. Present HF coastal radars that have proven to be most effective and acceptable operate continuously, spectrally processing and averaging the data over periods from one to three hours. Sequencing the operation of six radar stations so all can use the same frequency, each for a period of 10 minutes, for example, means that each is on only one-sixth of the time; the drop in data quality would be significant.
Pulse-to-pulse interleaving could be applied to systems where either a short pulse or a sequence of coded pulses is radiated. The echo from a given station's emission is acquired for a given station over an interval corresponding to the time for a signal to travel to and from the most distant range cell from which data is expected. At the end of this time, the waveform emission would normally be repeated. In pulse-to-pulse interleaving, at the end of this period at Radar #1, instead of repeating, Radar #2 transmits its waveform while Radar #1 remains silent (as well as any additional radars operating on that frequency). Next, Radar #3 radiates. Enough guard time is allowed to account for the propagation distance between the different radar sites. This method has an even more severe disadvantage than station sequencing. The time at Radar #1 until its next emission increases proportionately with the number of stations to be interleaved (plus some additional for the guard zones). This decreases the total energy emitted by each radar accordingly. But the signal-to-noise ratio depends directly on the total energy emitted from the radar. Hence, the maximum distance decreases within which high quality echoes can be obtained for a given power level radiated at the radar. This is too costly a penalty to pay.
One unsatisfactory approach, used to address the above mentioned problem, is normal frequency multiplexing. The problem with this method is that the spectral spacing between each frequency must be at least as great as the signal bandwidth. For six radars, this in effect is the same as requesting six separate frequencies, each with a 150 kHz wide channel (to follow the above example).
Another principal obstacle with current technology is the inefficient use of spectral bandwidth resources by multiple radar transmitters and receivers, and inability to allow each radar to operate at maximal efficiency as though none of the other radars were present.
Heretofore, the requirements of an efficient use of radar resources, and the ability to operate multiple radars at a maximum efficiency referred to above have not been fully met. What is needed is a solution that simultaneously addresses these requirements.